The present application relates to and incorporates herein by reference Japanese Patent Application No. 2000-386850 filed on Dec. 20, 2000.
1. Field of the Invention
The present invention relates to a flowmeter for detecting a fluid flow rate.
2. Related Art
A thermal flowmeter is known as a device used for detecting the flow rate of inlet air in the internal-combustion engine of a vehicle or the like. In the inlet system of the vehicle, the intake of air pulsates at long periods when the engine operates at a low speed under a heavy load. If the period of the intake pulsation matches the opening period of an inlet valve and an exhaust valve, the inlet air may flow upstream via the inlet valve.
JP-A-H6-160142 proposes a thermal flowmeter which detects not only the rate but also the direction of inlet air flow. In the flowmeter, two resistive elements for detecting the flow rate are provided on the upstream side and the downstream side of a heating resistor, respectively. The direction of the flow is detected based on the difference between temperatures detected by the respective resistive elements.
If the resistive elements are arranged on the upstream side and the downstream side of the heating resistor like the flowmeter according to JP-A-H6-160142, a thermal conductor such as a Si3N4 film whose area is relatively large should be employed for heat exchange among the inlet air, the heating resistor and the resistive elements. Therefore the detection sensitivity and responsiveness of the flowmeter are relatively low, because the heat capacity of the thermal conductor is relatively high. Further the air heated by the heating resistor surrounds the resistive element on the downstream side, and therefore the temperature of the resistive element varies only slightly due to the temperature of the inlet air. Accordingly the detection sensitivity of the flowmeter is still relatively low.
JP-A-2000-193505 (U.S. application Ser. No. 09/421086) also proposes a thermal flowmeter which detects the rate and the direction of a fluid flow. The flowmeter includes a resistive element for detecting the flow rate only on the upstream side of a heating resistor, and takes advantage of variation in the temperature distribution in the heating resistor to detect the rate and the direction of the flow. Therefore the flowmeter can employ a thermal conductor which has a relatively low heat capacity, and consequently its detection sensitivity and the responsiveness are improved.
However, the resistance of the heating resistor may vary due to migration across the ages, and the variation in the resistance is often reflected in the output characteristics of the flowmeter. Further dust or dirt laid on the flowmeter cannot be removed by passing a high current through the heating resistor, because the resistance of the heating resistor varies in response to the high current. Therefore the output characteristics of the flowmeter further vary across the ages due to the dust or dirt laid thereon.
It is an object of the present invention to provide a flowmeter which can detect the flow rate of a fluid precisely whether the fluid flows in the normal direction or the reverse direction, and whose output characteristics vary only slightly across the ages.
It is another object of the present invention to provide a flowmeter in which dust or dirt laid thereon can be removed by heating.
A flowmeter according to the present invention includes fluid temperature detection means, a heating resistor, flow rate detection means, heat temperature detection means, detection means, and control means. The fluid temperature detection means detects the temperature of a fluid flow. The flow rate detection means is arranged on the upstream side or downstream side of the heating resistor with reference to the fluid flow. The heat temperature detection means detects the temperature of the heating resistor.
The control means controls the temperature of the heating resistor so that the temperature detected by the heat temperature detection means approaches a reference temperature determined based on the temperature detected by the fluid temperature detection means. As a result, the temperature of the flow rate detection means varies depending on the rate and the direction of the fluid flow. The detection means detects the rate and the direction of the fluid flow based on the temperature of the flow rate detection means.
Preferably, the heating resistor has a plurality of protrusions which are serially connected by turning-back portions and extend in a direction, and each of the protrusions includes two elongated portions and a turning-back portion connecting therebetween. Thus the heating resistor is formed so as to have a predetermined width in the direction parallel to the fluid flow. The flow rate detection means is arranged so that the temperature of the flow rate detection means is lower than the reference temperature when fluid flows from the flow rate detection means to the heating resistor and higher than the reference temperature when fluid flows from the heating resistor to the flow rate detection means. The detection means detects the rate and the direction of the fluid flow by comparing the temperature of the flow rate detection means with the reference temperature or the temperature detected by the fluid temperature detection means.